Packs of sheets which are held together in this way by a coil made of metal or plastic wire are used, for example as shorthand pads and calenders. The production and the insertion of the coils take place in a so-called spiralling unit by means of a winding tool which is accelerated with the aid of a braking motor, i.e., an electric motor which can be switched very frequently between an `off` condition in which it is at a standstill and an `on` condition in which it rotates at its rated speed of about 6500 revolutions per minute. The motor winds the coil and inserts it into the pack of sheets with the aid of pivotal guide rollers. The winding tool is then braked to a standstill by the braking motor, the spirals cut to length and both ends bent back. This mode of operation is described in German Pat. No. 1,008,708.
As already mentioned in German Pat. No. 19 44 223, the performance, i.e. the number of strokes, of this known apparatus cannot be increased at will. As the winding tool can only operate during the time in which the pack of sheets is available in the spiralling unit and, moreover, time has to be allowed for cutting and bending back the ends the actual spiralling has to be carried out in a very short time. In practice, the processes described above are controlled by a timing shaft, which performs one revolution per operating process. There are only about 200.degree. available on the timing shaft, i.e. only about 55% of the clock time, for the actual formation of the coil, i.e. the starting up of the winding tool, the formation and the insertion of the coil and the braking of the winding tool. This means that only just over a second remains for the spiralling operation with a clock time of the machine of 30 operations per minute. Although the proportional spiralling time could be increased to 240.degree. on the timing shaft and thus the clock time to 35 operations per minute by reducing the other times in the intermediate period, a limit would be reached which restricted the performance of the entire apparatus and therefore sometimes an entire production line.
It is also mentioned in German Pat. No. 19 44 223, that attempts to increase the operating speed by means of electric motors having higher performances have not been successful because the advantages of their high performance are cancelled out by their high moment of inertia. Moreoever, tests have already been carried out using mechanical or electromagnetic couplings which turn on a continuously running drive means. These have not allowed adequate working lives to be obtained owing to the high amount of wear.
German Pat. No. 19 44 223 was therefore based on the mechanically very simple direct insertion of the coil into the pack of sheets and the spirals were produced continuously, then stored in the intermediate period and wound into the block by a special insertion device. Although higher operating speeds can be achieved in this way, this is at the expense of a mechanically more complicated system.